AC motors are widely used in many applications, including consumer applications such as washing machines, dish washers, electric fans, and automotive applications such as window lift control, electrical power steering systems, electromechanical brake systems and the like.
AC motor systems typically comprise a motor comprising a stator and a rotor and a motor controller to control the power supplied to drive the motor. In order to ensure good control of the motor, for example in order to meet specified motor performance requirements, the motor controller is required to know the position of the motor rotor.
Position sensors, such as position and velocity transducers, and the cabling and connectors required for such position sensors, increase the size, weight and complexity of the AC motor system and have also been a source of failure for AC motor systems. In order to eliminate such position sensors, particularly for small low cost motor controllers, much research has taken place into sensorless techniques for determining rotor position for different classes of motors under a variety of different operating conditions.
A simple technique uses the induced back electromotive force (EMF) generated in the motor. However, at rotor standstill or low speed there is insufficient back electromotive force (EMF) generated in the motor to enable an accurate estimate of rotor position.
More complex techniques are based upon injection of appropriate reference signals and the tracking of the response of the AC motor to the injected reference signal in order to determine the rotor position. The basis for most low and zero speed sensorless control techniques is the presence of a difference in the d-axis and q-axis characteristics of a motor: the d-axis and the q-axis define the dq rotating reference frame. This difference is used to determine the rotor position and is referred to as saliency. The motor characteristics may include for example inductance, or resistance. A salient motor is a motor that exhibits saliency, for example, a difference in inductance in the d-axis and q-axis depending on the position of the rotor. In a Permanent Magnet (PM) motor, there are several sources of saliencies, for example, rotor inherent saliency, saturation based saliency (stator, teeth).
Typically, as described for example, in US patent application no. 2006/0061319 and U.S. Pat. No. 6,894,454, a high frequency carrier signal is injected into the stator by combining the carrier signal with the reference voltage signal that controls the power provided to the stator of the AC motor. The resulting high frequency components, which carry the saliency position information and which are part of the feedback current from the stator, are then processed by a processor in the motor controller to determine the rotor position. The feedback current is also fed back as part of a control loop in the controller to control the power applied to the stator.
High frequency harmonic components are generated in the motor due to the carrier signal and the reference voltage signal and the operation of the control loop, for example during changes in motor load. Known sensorless control methods do not take account of the interference caused between the high frequency components carrying the saliency position information with the high frequency harmonic components generated in the control loop. Such interference impacts the performance of the motor controller in determining the rotor position.
Thus, there is a need for an improved motor controller.